(1) Field of the Invention
The present invention is directed toward membrane leaf packets, spiral wound modules and methods for making and using the same.
(2) Description of the Related Art
Membrane leaf packets are a component of many types of spiral wound modules. Such leaf packets comprise a folded membrane sheet which may optionally include a feed spacer positioned between opposing membrane leaves of the packet. Spiral wound modules are typically made by winding one or more membrane leaf packets and permeate spacer(s) about a permeate collection tube. The step of folding a membrane sheet during construction of the membrane leaf packet often results in the formation of cracks or other defects which can lead to leaks and/or contamination. A variety of techniques for mitigating these deleterious effects have been proposed.
U.S. Pat. No. 4,842,736 highlights the limited effectiveness of tapes positioned along the front or back side of the membrane sheet along the fold. Such tapes are described as being prone to delaminating during module use or cleaning. This reference further describes a preferred use of sealants (e.g. urethanes) or a soft melt plastic ribbon applied to the back side of the membrane sheet in the fold area. Such sealants penetrate and seal the interstices or voids of the back side of the membrane sheet.
A known variant of the preceding technique is to cover the sealant on the back side of the membrane sheet with a plastic strip (e.g. Mylar). The plastic strip protects the sealant until cured.
U.S. Pat. No. 7,303,675 and US 2007/0068864 both disclose the application of sealants (e.g. urethanes) to the front side of the membrane sheet along the fold area.
U.S. Pat. No. 5,147,541 highlights the shortcomings of tapes (delaminating) and sealants (slow cure time) and describes an alternative means for reinforcing the fold comprising the application of a reinforcing strip to back side of the membrane sheet along with the application of heat and pressure to front side to “densify” and fuse layers in the fold area.
U.S. Pat. No. 7,335,301 highlights the shortcomings of “fusing” techniques; namely the length of time required to fuse the layers of the membrane sheet and proposes the use of UV curable sealants applied to the fold area.
U.S. Pat. No. 5,681,467 describes the use of thermal energy or swelling solvent to soften fold area prior to folding operation.
Due to its relative thickness (as compared with the leaf portion of a leaf packet), the bulk of the fold area limits the number of membrane leaf packets that can be effectively located about the permeate tube. Moreover, the fold of the leaf packet may limit the manner in which spiral wound modules are manufactured. For example, the use of exposed uncured sealant on the back side of the membrane sheet may limit the use of automated winding processes. That is, once applied to the fold the uncured sealant may be easily removed or “smeared” during the assembly of the spiral wound module. While the use of sealants on the front side of the membrane sheet may avoid this concern, sealants applied to the front side often result in tearing of the membrane surface once the sealant is cured. Unfortunately, tapes tend to delaminate from the front side of the membrane sheet.
Despite the known techniques for reinforcing the fold area of membrane leaves, each approach has shortcomings. An improved approach is desired which is adaptable to automated leaf and/or module manufacturing processes. Moreover, an improved fold reinforcing approach is desired which permits a greater number of membrane leaves to be effectively located about a permeate tube. The entire content of each of the aforementioned patents and patent applications are incorporated herein by reference.